Photoelectrochemical cells (PECs) based on a mesoporous, nanocrystalline metal oxide film sensitized with organic or organometallic dyes have been studied extensively for the past twenty years as a potential low cost alternative to more traditional, solid state photovoltaics. PECs include those typical device architectures known in the art such as those described in, for example, O'Reagan et al., Nature, Vol. 353, pp. 737-740, October 24, the content of which is incorporated by reference. An exemplary PEC may have an organic, organometallic or inorganic chromophore adsorbed on the surface of nanocrystalline metal oxide film, which forms a photoactive electrode. The second electrode consists typically of metal, such as platinum. The charge transport between the electrodes is facilitated by a liquid electrolyte comprising solvent and a charge transporting material.
Significant progress has been made in optimization of the components of the dye sensitized solar cell (DSSC) with highest reported efficiencies currently exceeding 11%. As part of search for new approaches to further improvement in efficiency over past several years, a number of research groups reported studies of PECs in which the sensitizing dyes are substituted with quantum confined semiconductor nanocrystals, also called nanocrystal quantum dots (NQDs) of materials, such as InP, CdS, CdSe, CdTe, PbS and InAs. In studies of these PECs, also called quantum dot sensitized solar cells (QDSSCs) it was demonstrated that NQDs can function as efficient sensitizers across a broad spectral range from the visible to mid-infrared, and offer advantages such as the tunability of optical properties and electronic structure by simple variation in NQD size, while retaining the appeal of low-cost fabrication. In addition, as was demonstrated recently, NQDs of certain materials (e.g., PbSe, PbS) have the ability to efficiently convert the energy of a single photon into multiple electron-hole pairs via a process called carrier multiplication (CM) or multiple exciton generation (MEG). Provided that carriers generated by the MEG effect can be effectively extracted from NQDs, this process has the potential to significantly improve the efficiency of QDSSCs.
Two distinct approaches to the sensitization of metal oxide (MOx) films with NQDs have been demonstrated in recent studies. In one approach, NQDs are generated on the surface of MOx films in-situ, using chemical bath deposition (CBD) or successive ionic layer adsorption and reaction (SILAR). The advantage of the in-situ deposition approaches are their simplicity, the fact that the NQDs are in direct electronic contact with MOx, and that they can easily produce MOx films with high surface coverage of the sensitizing NQDs. However, there are several limitations of the in-situ approaches, such as poor control over NQD chemical composition, crystallinity, size and surface properties, which are likely to limit effective exploitation of the advantages of the NQDs, such as MEG effect.
An alternative approach to development of these materials that include MOx films and NQDs is based on a two step process, whereby NQDs are first independently synthesized using established colloidal synthesis methods and the MOx film is subsequently sensitized by exposure to a concentrated solution of the NQDs. The advantage of this approach is a significantly higher control over the chemical, structural and electronic properties of the NQDs compared to the in-situ approaches.